Oxygen measuring probe

ABSTRACT

An oxygen measuring probe for use with molten metal baths for the manufacture of glass which probe comprises a probe body (4) comprising an elongate tubular member (6) closed at one end by a separate tip part (8) formed of stabilised zirconia which constitutes a solid electrolyte through which oxygen ions can pass, said tubular member (6) being formed of a heat-resistant material different from said zirconia, a first electrode (16) connected to the inner surface of the zirconia tip part (8), earthing means adapted for connection to earth out of contact with the molten metal and voltage measuring means (24) connected between the first electrode (16) and said earthing means (26).

FIELD OF THE INVENTION

This application is a division of application Ser. No. 08/357,788, filedon Dec. 16, 1994, now U.S. Pat. No. 5,480,523, which is a continuationof application Ser. No. 08/035,786, filed on Mar. 23, 1993, nowabandoned.

The present invention relates to an oxygen measuring probe for use withmolten metal baths for the manufacture of glass, and to a method of useof such a probe.

BACKGROUND OF THE INVENTION

In the float glass process in which a bath of molten tin or tin alloy isused to support an advancing ribbon of glass, oxygen is a majorcontaminant. Oxygen is believed to be responsible directly or indirectlyfor reducing glass quality in several ways, giving rise to poor bloomgrades, CO bubble, tin speck faults and for tin pick-up faults.

In the float glass process the bath atmosphere is controlled, generallyby maintaining an atmosphere of nitrogen and hydrogen. In practicalterms it is impossible to exclude oxygen completely from the bath;oxygen can enter the bath by various pathways, such as through the exitseal, through leaks in windows and side seals, as a contaminant of theatmosphere supply, or with the glass itself as dissolved oxides (e.g.SO₂ and H₂ O). A series of interactions then take place with hydrogen inthe atmosphere, with the tin, and with the glass itself. It is noteconomic to attempt to reduce the contamination below a particularlevel. Ingress can, however, be reduced to a level at which itsdeleterious effects are of little or no consequence.

It is clearly important to know the amount of oxygen present so that ifthis rises, appropriate steps can be taken to prevent furthercontamination, whilst the glass subject to contamination and the levelof contamination can be readily identified.

The oxygen content of the tin can be measured by conventional analyticaltechniques; by removing a sample, reacting with carbon under vacuum andmeasuring the carbon monoxide released. This is however a lengthyprocedure which demands great skill and care if the required degree ofprecision is to be achieved; as a result, this measurement is rarelyperformed.

The chemical state of the bath with respect to oxygen contamination canalso be monitored on a regular basis using indirect means such as anatmosphere extractive technique, measurement of the tin count, ormeasurement of the bloom grade. The extractive technique indicates thelevel of contamination of the atmosphere but does not necessarily sayanything about the level of contamination in the tin. Further, being anextractive technique, the sample lines are prone to blockage. The tincount and bloom grade are measurements made on the glass, and areindicative of the amount of tin present in the surface, which isdirectly related to the level of oxygen contamination. Since these testsare carried out on the product there is necessarily a time delay and theresults give little indication of the distribution of contaminationwithin the bath.

A known technique for measuring the oxygen content of the molten tin onan in situ basis uses a measuring probe located in the bath so as toextend into the molten tin. This probe is the subject of U.S. Pat. No.3,625,026. The probe comprises a tubular body of zirconia which has beendoped to induce conductivity to oxygen ions and which therebyconstitutes a solid electrolyte. Electrical connection is made to theinside of the tube, and directly to the molten tin which as a conductorconstitutes an electrical connection to the outside of the tube. Agalvanic cell is thereby effectively set up, resulting from the oxygenconcentration internally of the tube which is separated from the oxygenconcentration in the molten tin by the solid zirconia electrolyte. Thecell emf is indicative of the oxygen concentration at the outer side ofthe probe. By supplying a gas of constant oxygen concentration to theinside of the tube an absolute value for the oxygen concentrationexternally of the tube can be determined.

Various problems exist with this probe. The probe body of zirconia isparticularly fragile, and is sensitive to thermal shock arising oninsertion of the probe into the molten tin, which may typically be at700° C. Zirconia has a high thermal coefficient of expansion, such thaton this initial insertion, considerable stresses are set up within thezirconia body, rendering the probe liable to fracture. There is afurther thermal effect which arises on more prolonged usage which cancause the probe to fracture. The stabilisation of the zirconia by theaddition of dopants causes the zirconia to take a particular crystallineform (specifically, a cubic tetragonal form). Although this form isstable at the temperature of the molten tin, at lower temperatures,below about 400° C., the stable form of zirconia is a differentcrystalline form (specifically, a cubic monoclinic form), and onprolonged usage a transition to this form will occur. A substantialtemperature gradient exists along the length of the probe when in use,such that the temperature conditions in which the second crystallineform is the stable form are generally found at a region of the proberemote from the molten tin. This change of crystalline structureinvolves a volume change, such that a junction region between the twoforms of zirconia will constitute a particular site of stress at whichthe zirconia body is liable to fracture.

SUMMARY OF THE INVENTION

The present invention seeks to provide an oxygen measuring probe whichovercomes these problems.

In order to derive an accurate measurement of the oxygen concentration,in addition to obtaining a value for the cell emf, one requires a valuefor the cell temperature. Such a value can be obtained from separatelymeasuring the molten metal temperature. It has also been proposed tomeasure the probe temperature with a thermocouple extending part waywithin the probe. Neither of these means ensure accurate temperaturemeasurement of the cell.

According to one aspect of the invention there is provided a oxygenmeasuring probe for use with molten metal baths for the manufacture ofglass, which probe comprises: a tubular probe body comprising anelongate tubular member closed by a separate tip part which is connectedthereto, which tip part is formed of stabilised zirconia whichconstitutes a solid electrolyte through which oxygen ions can pass, saidtubular member being formed of a heat resistant material different fromsaid zirconia; and an emf measuring device for measuring the emfgenerated in use between inner and outer surfaces of the zirconia tippart, wherein a thermocouple disposed in contact with an inner surfaceof the zirconia tip part is provided for measuring the temperature ofthe zirconia tip part.

By providing a thermocouple which is in contact with the zirconia tippart one obtains a direct measurement of the actual cell temperature. Byproviding only a tip part of zirconia this will rapidly reach in itsentirety the high temperature in or above the bath of molten metal, andso the risk of failure due to thermal shock is reduced. Furthermore,when the probe is inserted in the molten metal with the entire tip partimmersed, the entire tip part will be held at a temperature above thatat which a change of crystalline form could occur.

A further advantage arising from the probe structure of the invention isthat prior to assembly, the inside surface of the zirconia tip part canbe easily accessed. The unitary zirconia body of the known probe is verylong and thin such that access to the inside of the tip is verydifficult, and it is therefore very difficult to form a good electricalconnection of the electrode to the inner surface of the tip.

A further difficulty which arises is that it is problematic to securethe electrode to the inner surface of the zirconia tip part, and where athermocouple is additionally provided this problem also applies to thethermocouple wires. Preferably, the emf measuring device, and thethermocouple include a common electrode in the form of a metallic wireconnected to an inner surface of the zirconia tip part, the thermocouplefurther comprising an additional thermocouple wire of differentcomposition connected to the first electrode at the tip part to form athermocouple junction therewith.

Preferably, with the present probe the first electrode wire and theadditional thermocouple wire are connected to the inner surface of thezirconia tip part by means of a cement of platinum applied as a paste.This gives a particularly secure and effective electrical and thermalcontact with the zirconia tip part. The common electrode may be formedof platinum, and the additional thermocouple wire formed of a platinumalloy.

In an embodiment adapted specifically for measuring the oxygenconcentration in the atmosphere above the tin bath, the emf measuringdevice comprises a first electrode connected to an inner surface of thezirconia tip part, a second electrode connected to the outer surface ofthe tip part, and a voltmeter connected between the first and secondelectrodes. In this case both first and second electrodes are secured tothe tip part by means of a cement of platinum applied as a paste.

A further problem encountered with the known probe arises from the factan electrical connection must be made directly into the molten tin.Connection has previously been made by a platinum wire having a lengthof rhenium wire welded to the end of this platinum wire, with only therhenium extending into the molten tin. Rhenium is used because, unlikeplatinum, it is not attacked by the molten tin, and is relatively stabletowards oxygen at the concentrations which generally exist in floatglass baths. However, if a severe oxygen contamination occurs, therhenium can become oxidised, whereby the electrical connection isdisrupted.

According to a further aspect of the present invention there is providedan oxygen measuring probe for use with molten metal baths for themanufacture of glass, which probe comprises an elongate tubular bodyclosed at a measuring end thereof for insertion into the molten metal,at least a portion of the body at the measuring end being formed ofstabilised zirconia, which constitutes a solid electrolyte through whichoxygen ions can pass, a first electrode connected to an inner surface ofthe zirconia portion, earthing means adapted for connection to earth outof contact with the molten metal, and voltage measuring means connectedto the first electrode and to said earthing means.

With this arrangement there is no reliance on direct electricalconnection to the molten metal. The arrangement uses the observationthat the molten metal is effectively an earth, so that measurement ofthe emf between the probe electrode and a direct connection to earth iseffectively equivalent to measurement between the probe electrode andmolten metal which is in contact with an outer side of the zirconia,subject to correction of any emf's arising from junctions in theelectrical connections earth. This modification also simplifiesconstruction and reduces the cost of the probe.

The tubular probe body preferably comprises an elongate tubular memberwhich is closed at the measuring end by a separate tip part ofstabilised zirconia, sealed to the tubular member, said tubular memberbeing formed of a heat-resistant material which is different from saidzirconia.

Preferably, the elongate tubular member is formed of alumina, and thezirconia tip part is sealed to the alumina tube by a glass ceramicmaterial having a thermal expansion coefficient intermediate betweenthat of alumina and zirconia. The use of a glass ceramic material ofmatched thermal expansion provides an effective non-porous seal, whichis able to absorb to some extent the thermal expansion of the zirconiatip on initial insertion into or location adjacent to the molten metal.

Preferably, the zirconia tip part comprises a generally annular portionreceived within an end of the tubular member, at which the tip part issealed to the elongate tubular member, and a hollow relatively shortoutwardly convex portion depending therefrom. Conveniently the outwardlyconvex portion may be of substantially conical shape, the connection ofthe first electrode being made within the tip part in the region of theapex thereof. The shortness and internal shape of the zirconia tip partfacilitites appropriate location of the platinum paste and electrode onassembly of the probe.

The probe preferably also includes means for direction of anoxygen-containing reference gas onto the inner surface of the zirconiatip part. Maintaining a known concentration of oxygen within the probetip is essential where an absolute value for oxygen concentration on theoutside of the probe tip is required.

The invention in a further aspect also resides in a method of measuringthe oxygen concentration in molten metal with a probe as definedhereinabove, wherein the probe is inserted into the molten metal andsaid earthing means are connected to earth out of contact with themolten metal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described, by way of exampleonly, with reference to the following drawings in which:

FIG. 1 is a schematic view of a first embodiment of a measuring probe inaccordance with the invention, adapted for measuring the oxygenconcentration in molten tin; and

FIG. 2 is a schematic view of a second embodiment of a measuring probein accordance with the invention, adapted for measuring the oxygenconcentration in the atmosphere above a bath of molten tin.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIG. 1 of the drawings, a first embodiment of an oxygenmeasuring probe 2 is shown, the probe comprising a tubular probe body 4closed at one end which constitutes a measuring end, which in use isimmersed in a bath of molten tin of a float glass bath. The probe body 4comprises a cylindrical tubular member 6 formed of a heat-resistantrefractory material, such as alumina, which is closed at its lower endby a tip part 8 formed of zirconia (zirconium oxide). The zirconia isstabilised by the addition of one or more of CaO, MgO and Y₂ O₃ as isknown in the art, whereby the zirconia constitutes a solid electrolytewhich allows conduction of oxygen ions therethrough. The zirconia tippart 8 comprises an annular portion 10 at which the tip part 8 isattached to the tubular part 6, and a hollow generally conical portion12 depending therefrom.

The tip part 8 is connected to the tubular member 6 by means of a glassceramic material 14 which forms a non-porous seal therebetween. Theglass ceramic material is arranged to have a coefficient of thermalexpansion intermediate between that of zirconia and alumina. Thismatched expansion glass ceramic seal is formed according to the methodof Rogers, Butler and Steele as disclosed in J. Sci. Inst. (J. PhysicsE) 1969 Ser. 2 volume 2 page 102, although in the present case the sealis formed between alumina and zirconia, instead of between a alumina andmetal as in the reference.

A reference electrode 16 comprises a wire, formed preferably ofplatinum, extending within the probe body 4 along the length thereof,which is electrically and thermally connected to the inner surface ofthe tip part 8 at the inner apex thereof.

A particularly effective connection can be obtained by cementing theelectrode to the zirconia tip by means of a platinum paste, indicated at17. The paste comprises a suspension of platinum in a viscous organicmedium which is applied to the inside of the tip part with the electrodeembedded therein, and then fired to volatilise the organic medium,leaving the electrode cemented to the tip part.

It is intended that on assembly of the probe, the connection of theplatinum wire electrode 16 is made to the tip part 8 prior to theconnection of the tip part 8 to the tubular member 6, since easy accessis then possible.

An internal annular tube 20 of refractory material, preferably alumina,surrounds the electrode wire 16. This internal tube 20 forms apassageway for the supply of a reference gas containing a known amountof oxygen, typically air, to the inner surface of the tip part 8.Maintenance of a known oxygen concentration at the inner surface of thetip is necessary where it is required to calculate an absolute value foroxygen concentration in the molten tin, as is apparent from thecalculations shown below. It is also highly desirable to maintain aconstant oxygen concentration internally where only relative changes ofemf are used to indicate relative change of oxygen concentration in thebath. The tube 20 also serves to protect the platinum electrode 16 fromthe effects of heat.

A further wire 22 extends within the probe body 4 and is connected tothe electrode wire 16 where this is cemented to the tip part 8, to forma thermocouple junction with the electrode wire 16. Thus, the electrodewire 16 forms a common electrode wire for both the voltage measurementand temperature measurement. This simplifies the design and assembly.The wire 22 is preferably formed of a platinum alloy, for example a 13%Rh/Pt alloy. A millivoltmeter 23 provided externally of the probe body 4is connected between the wire 16 and wire 22; measurement of thethermoelectric emf developed between the electrode wire 16 and wire 22enables a value for the temperature at the tip to be obtained, whichtemperature value is necessary for a calculation of an absolute valuefor oxygen concentration in the molten tin. Tube 20, electrode 16 andthermocouple wire 22 are supported in fixed positions at an upper end ofthe probe housing 4 within a sleeve member (not shown).

Also externally of the probe body 4, the reference electrode 16 isconnected to a millivoltmeter 24 which is in turn connected to ameasuring electrode 26. This measuring electrode 26 is in turn connectedto earth. Typically, a metal portion of the bath casing or unpaintedwater pipe provides the earth. The electrode 26 may, but need not, be ofplatinum. In practice the platinum of electrode 16 may terminate at theprobe head and have a lead to the meters of a different material, theelectrode 26 also being of that different material. Use of the sameelectrode material for the electrodes on both sides of themillivoltmeter obviates the need for correction of the measurement ofemf resulting from the electrode/millivoltmeter junction; however, acorrection to the measured emf is required as a result of an emfgenerated by the junction of the measuring electrode 26 and earthreturn. The millivoltmeter is a high impedence millivoltmeter, forexample of input impedence 10¹³ ohms, whereby polarisation of the cellis avoided.

In use, the probe is positioned with the tip part 8 entirely immersed inthe molten tin. The tin level is indicated in FIG. 1 by referencenumeral 27. The entire tip part 8 therefore remains at a temperatureabove that at which a change of crystalline form could occur. The regionwhere the temperature conditions exist which would cause the change ofcrystalline form in zirconia are found at the part of the probe formedof alumina, in which no such change occurs and so presents no thermalstress problem. Furthermore, the surface of the molten tin, which alsoconstitutes a site of thermal stress, lies in the region of the aluminatubular member 6, which material is considerably less susceptible tothermal stress-induced fracture than zirconia. Air is directed at theinner surface of the tip part 8 via the tube 20, and measurements takenof the emf shown by the voltmeter 24, which emf is indicative of theoxygen concentration in the molten tin.

The relationship between the measured emf and oxygen concentration inthe molten tin is derivable as follows:

Oxygen reacts with tin according to the equation: ##STR1##

By Mass Action Law: ##EQU1## where K=Equilibrium constant a Sn=Activityof tin, taken as 1

a (O)_(Sn) =Activity of Oxygen in tin

from thermodynamic tables: ##EQU2## where T=absolute temperature

The activity of oxygen in tin is defined as follows: ##EQU3##

The saturated concentration of oxygen in tin is given in the reference"Thermodynamics and Solubility of oxygen in liquid metals Part 2--Tin,T. N. Belford and C. B. Alcock TFS 61,443 (1965)" as:

    C.sub.s (ppm)=1.345×10 (-5730+4.19/T)

Thus we can relate the oxygen partial pressure over a solution of oxygenin tin to the oxygen concentration as follows: ##EQU4##

The Nernst equation relates this partial pressure to the Emf as follows:##EQU5## if E=Emf developed in millivolts R=Universal gas constant

F=Faraday constant

Reference Gas=Air

P₀.sbsb.2 reference=0.21 atmospheres

then ##EQU6##

The above relation ignores any error which exists as a result of an emfgenerated by the platinum wire 26/earth return couple. Where onlychanges in oxygen concentration in the tin are required to be measured,it is not necessary to correct for this thermoelectric emf. However,this may be done readily by modifying the constant appropriately:

The thermoelectric emf generated by the platinum wire/earth returncouple (E') may be represented by the equation:

    E'=0.01025×T-6.45

then, the emf E in equation 1 above is replaced by (E+E').

A modification of the measuring probe of FIG. 1 allows the measurementof oxygen concentration in the atmosphere above The tin bath. Thismodified probe is shown in FIG. 2, in which like numerals are used toindicate like pans. In this case, since the zirconia tip part 8 whichconstitutes the solid electrolyte is not immersed in the molten tin,electrical connection to the outer surface of the tip must be provided;the measurement of the emf generated by the cell requires directmeasurement of the potential at the inner and outer surfaces of thezirconia tip part 8. This connection is preferably in the form of aplatinum wire 30 which is connected to the millivoltmeter 24 and to alength of rhenium wire 32 at a welded joint 34. The rhenium wire 32 iscemented by platinum paste at 36 to the outer surface the zirconia tippart 8, opposite the internal connection of the electrode 16, in themanner as described above in relation to the connection of the electrode16. A further alumina tube 38 is used to enclose the platinum wire 30and upper portion of the rhenium wire 32 to protect these from theatmosphere.

In this case, the Nernst equation can be used to relate the emfdeveloped to the oxygen concentration in the atmosphere as follows:##EQU7## If E=emf developed by probe in millivolts R=Universal gagsconstant

F=Faraday constant

T=Absolute temperature

Reference gas=Air

P₀.sbsb.2 reference=0.21 atmospheres ##EQU8##

Here, since the bath atmosphere contains hydrogen, the platinum cementon the outer surface of the probe tip 8 catalyses a reaction betweenhydrogen and oxygen, and the voltage generated by the measuring probewill relate not to free oxygen, but to that which would be present atequilibrium.

What we claim is:
 1. An oxygen measuring probe for use with molten metalbaths for the manufacture of glass, which probe comprises: a tubularprobe body comprising an elongate tubular member closed by a separatetip part which is connected thereto, which tip part is formed ofstabilized zirconia which constitutes a solid electrolyte through whichoxygen ions can pass, said tubular member being formed of alumina; andan emf measuring device for measuring the emf generated in use betweeninner and outer surfaces of the zirconia tip part, wherein the zirconiatip part comprises a generally annular portion received within an end ofthe tubular member, at which the tip part is sealed to the elongatetubular member by a glass ceramic material having a thermal expansioncoefficient between that of alumina and zirconia, and a hollow portiondepending therefrom having inwardly sloping walls which meet to definean apex at an end of the zirconia tip part remote from the tubularmember, to thereby define a substantially conically shaped portion andwherein a thermocouple is disposed in said substantially conicallyshaped portion of the tip part and is in contact with an inner surfacethereof for measuring the temperature of the zirconia tip part, saidthermocouple being located between said inwardly sloping walls and incontact with the inner surface of the tip part at the apex.
 2. Theoxygen measuring probe according to claim 1 wherein the emf measuringdevice and the thermocouple include a common first electrode in the formof a metallic wire which is connected to the inner surface of thezirconia tip part in the region of the apex thereof, and wherein thethermocouple further comprises an additional thermocouple wire ofdifferent composition connected to the first electrode at the tip partto form a thermocouple junction therewith.
 3. The oxygen measuring probeaccording to claim 2 wherein the first electrode wire and the additionalthermocouple wire are secured to the tip part by means of a cement ofplatinum applied to the tip part as a paste.
 4. The oxygen measuringprobe according to claim 2 wherein the first electrode comprises aplatinum wire, and wherein the additional thermocouple wire comprises aplatinum alloy wire.
 5. The oxygen measuring probe according to claim 2wherein the emf measuring device further comprises a second electrodeconnected to earth out of contact with the molten metal, and a voltmeterconnected between the first and second electrodes.
 6. The oxygenmeasuring probe according to claim 2 wherein the emf measuring devicefurther comprises a second electrode connected to the outer surface ofthe tip part, and a voltmeter connected between the first and secondelectrodes.
 7. The oxygen measuring probe according to claim 6 whereinthe second electrode is secured to the zirconia tip part by means of acement of platinum applied as a paste.
 8. An oxygen measuring probeaccording to claim 1 wherein gas supply means are provided for directionof an oxygen-containing reference gas onto the inner surface of thezirconia tip part.
 9. The oxygen measuring probe as claimed in claim 1,further comprising a first electrode connected to the inner surface ofthe zirconia tip part, earthing means connected to earth out of contactwith the molten metal, the emf measuring device being connected to thefirst electrode and to said earthing means, whereby in use, the moltenmetal acts as an earth such that the voltage indicated by said emfmeasuring device represents the voltage generated across the solidelectrolyte.
 10. The oxygen measuring probe according to claim 9 whereinelectrical connections to the emf measuring device from the firstelectrode and from the earthing means are of the same material.
 11. Theoxygen measuring probe according to claim 9 wherein a wire of platinumalloy extends within the tubular member and is connected to the firstelectrode at the apex of the zirconia tip part to form a thermocouplejunction therewith.
 12. An oxygen measuring probe for use with moltenmetal baths for the manufacture of glass, which probe comprises: atubular probe body comprising an elongate tubular member closed by aseparate tip part which is connected thereto, which tip part is formedof stabilized zirconia which constitutes a solid electrolyte throughwhich oxygen ions can pass, said tubular member being formed of a heatresistant material different from said zirconia; and an emf measuringdevice for measuring the emf generated in use between inner and outersurfaces of the zirconia tip part, wherein the zirconia tip partcomprises a generally annular portion received within an end of thetubular member at which the tip part is sealed to the elongate tubularmember, and a hollow portion depending therefrom having inwardly slopingwalls which meet to define an apex at an end of the zirconia tip partremote from the tubular member, to thereby define a substantiallyconically shaped portion and wherein a thermocouple is disposed in saidsubstantially conically shaped portion of the tip part and is in contactwith an inner surface thereof for measuring the temperature of thezirconia tip part, said thermocouple being located between said inwardlysloping walls and in contact with the inner surface of the tip part atthe apex, said emf measuring device and said thermocouple furtherincluding a common first electrode in the form of a metallic wire whichis connected to an inner surface of the zirconia tip part in the regionof the apex thereof, and said emf measuring device further comprising asecond electrode secured to the outer surface of the tip part by meansof a cement of platinum applied as a paste, and a voltmeter connectedbetween the first and second electrodes.
 13. The oxygen measuring probeaccording to claim 12 wherein the thermocouple further comprises anadditional thermocouple wire of different composition connected to thefirst electrode at the tip part to form a thermocouple junctiontherewith.
 14. The oxygen measuring probe according to claim 13 whereinthe first electrode wire and the additional thermocouple wire aresecured to the tip part by means of a cement of platinum applied to thetip part as a paste.
 15. The oxygen measuring probe according to claim13 wherein the first electrode comprises a platinum wire, and whereinthe additional thermocouple wire comprises a platinum alloy wire.
 16. Anoxygen measuring probe according to claim 12 wherein gas supply meansare provided for direction of an oxygen-containing reference gas ontothe inner surface of the zirconia tip part.